# What is Propagation Velocity in a Cable?

Cables

What is Velocity Factor (Vf) or velocity of propagation (Vp) in a cable?

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The Velocity of propagation (Vp) of a cable is the speed at which an electrical signal can propagate through the cable in comparison to the speed of light. For example, in a vacuum, the velocity of propagation would be 100% or 1 (depending on how it is represented). So, if the Propagation Velocity is 70%, it means that the signal in a cable will propagate at 0.7 times the speed of a signal if the dielectric of the cable was vacuum.

The Velocity factor or Propagation Velocity of a cable can be calculated by the formula below (where ϵ is the dielectric constant of the dielectric material used in the cable). The velocity of propagation or velocity factor is a coaxial cable is determined by the dielectric used. There are a number of materials that are used as the dielectric in a cable. Each of these dielectrics has a different dielectric constant and thus a different velocity factor. Here is a table of the most common dielectrics used in coax cables with their dielectric constant and velocity factor.

 Dielectric Material Dielectric Constant Velocity Factor Velocity of Propagation Polyethylene (PE) 2.3 0.659 65.9% Foam Polyethylene 1.3 - 1.6 0.79 - 0.88 79% to 88% Air Spaced Polyethylene 0.84 - 0.88 84% to 88% Solid PTFE 2.07 0.695 69.5% Air Spaced PTFE 0.85-0.90 85% to 90%